Time-Division Duplexing (TDD) networks allow full duplex communication over a single frequency band. This is achieved by allocating a first set of timeslots to transmissions in a first direction (e.g. downlink) and a second set of timeslots to transmissions in a second direction (e.g. uplink). Two or more network nodes may then be configured to send and receive at the appropriate timeslots.
There are significant benefits to utilizing TDD, rather than its Frequency Division Duplexing (FDD) counterpart (which divides uplink and downlink into two frequency bands). For example, a TDD network may allocate a different number of timeslots to either the uplink or downlink directions, such that it may be tailored to asymmetric uplink and downlink data rate demands. FDD networks, on the other hand, do not fully utilize one of the uplink or downlink frequency bands for asymmetric scenarios. Thus, the TDD network offers greater spectrum usage compared to FDD.
A major design consideration when using a TDD network is aligning the uplink and downlink timeslots to avoid interference between transmissions. For example, if two TDD networks (each comprising a basestation and a User Equipment, UE), have both overlapping coverage areas and overlapping uplink and downlink timeslots, then downlink transmissions in the first TDD network would significantly interfere with uplink transmissions in the second TDD network. Therefore, all TDD networks using the same frequency band use a particular frame structure (i.e. a particular sequence of time slots, in which a first portion are allocated for uplink and a second portion are allocated for downlink, separated by transition points) and align the frames such that there are no overlapping uplink and downlink timeslots. This alignment process is known as “synchronization.” There are a number of synchronization techniques to minimize the chance of interference.
A first category of synchronization techniques involves the TDD network receiving timing signals over a backhaul connection. Each node in the TDD network may therefore receive the same timing signal from a remote reference clock (a “Primary Reference Time Clock”) and synchronize their uplink and downlink timeslots accordingly. Examples of backhaul synchronization include NTP, Synchronous Ethernet (SynchE) and IEEE-1588v2. These techniques provide sub-microsecond synchronization. However, there is an associated backhaul cost, and there are particular requirements specified by each protocol which make them unsuitable for small cell (e.g. femtocell) deployments.
A second category of synchronization techniques involves the TDD network receiving timing signals ‘Over-the-Air’ (OTA). This category includes both RF signal based techniques and also techniques using Global Navigation Satellite Systems (GNSS). GNSS synchronization can provide extremely accurate timing signals and is widely adopted by macrocell base stations. However, they are not well suited to an indoor deployment scenario (such as small cells), and they have an associated equipment cost for the GNSS processing modules.
OTA synchronization by RF signal based techniques generally relate to network nodes detecting synchronization signals in the network. A network node may then decode a timing signal from the synchronization signal and, after compensating for any propagation delay, may synchronize its downlink and uplink signals accordingly. There are no additional hardware costs involved. However, there are several issues with these techniques. Firstly, they are protocol dependent, so may not work effectively when different network nodes are provided by different network operators (this is particularly relevant for small cells, which will be deployed extensively by different operators and are likely to have overlapping coverage areas). Secondly, remote network nodes may not be able to receive the synchronization signal directly from a macrocell basestation. This may be addressed by relaying the synchronization signal across several nodes to the remote network node, but this introduces inaccuracies.
Furthermore, in the case of multiple-operator deployments of TDD networks, the operators typically use different frequency bands to avoid interference. However, this is not completely effective so they must still adopt the same frame structure and synchronize their time slots. Whilst this reduces interference, some operators may have to use a frame structure which is not ideally suited to their network demands.
It is therefore desirable to alleviate some or all of the above problems.